Liquid laser containing cycloheptatriene

ABSTRACT

A DYE LASER UTILIZING AN ACTIVE MATERIAL COMPRISING THE DYE RHODAMINE 6G IN SOLUTION WITH THE TRIPLET STATE QUENCHER CYCLOHEPTATRIENE. THE RESULTANT SOLUTION PROVIDES A DYE LASER HAVING RELATIVELY LONG HIGH ENERGY OUTPUT PULSES.

y 8, 1972 c. BRECHER ETAL 3,677,95

LIQUID LASER CONTAINING CYCLOHEPTATRIENE Filed Oct. 29, 1970 Q CI: 22

I2 f l4 'l6 HEAT F- EXCHANGER Fig.2.

INVEA'TORS CHARLES BRECHER ROMANO PAPPALARDO BY HAROLD SAMELSON A TTORNEY.

3,677,959 Patented July 18, 1972 3,677,959 LIQUID LASER CONTAINING CYCLOHEPTATRIENE Charles Brecher, Little Neck, Romano Pappalardo, Westbury, and Harold Samelson, Sea Cliff, N.Y., assignors to GTE Laboratories Incorporated Filed Oct. 29, 1970, Ser. No. 85,196 Int. Cl. F21k 2/02; H01s 3/00 U.S. Cl. 252-301.2 R 9 Claims ABSTRACT OF THE DISCLOSURE A dye laser utilizing an active material comprising the dye Rhodamine 6G in solution with the triplet state quencher cycloheptatriene. The resultant solution provides a dye laser having relatively long high energy output pulses.

BACKGROUND OF THE INVENTION The invention relates to liquid lasers and in particular to lasers, generally known as dye lasers, which utilize an active material comprising an organic dye in a solvent.

The recognition that certain fluorescent organic dyes in solution may be stimulated to emit coherent radiation has led to the development of the dye laser. Stimulated emission is produced by excitation of the dye solution with electromagnetic radiation of the proper wavelength and having an energy density which is above a critical level. Part of the radiation incident on the solution is absorbed by the dye molecules therein causing the energy level of the dye molecules to be raised from a ground to an excited singlet state. When the concentration of molecules in the excited singlet state reaches a value called the critical inversion, coherent laser emission occurs. The emission results from a loss of energy as the dye molecules return to the ground state.

The properties of the dye laser make development of a practical laser of particular interest. In contrast with other lasers, the dye laser emits energy over a relatively broad portion of the electromagnetic spectrum. Dye lasers can then be turned to emit energy at a specified wavelength within this broad portion of the spectrum by relatively simple techniques. The large number of organic dyes available permits use of a dye which will emit energy within a selected portion of the spectrum thereby allowing flexibility in the use of these 1 asers. Use of a liquid active medium permits the medium to be circulated thereby improving the optical properties of the medium and permitting rapid successive excitations of the laser.

Development of a practical dye laser has been impeded by a phenomenon known as early extinction or early termination. Theoretically, as long as the power level of the laser excitation source is above the critical level the laser will emit coherent radiation. However, it has been found that the laser output is actually quenched, or reduced to zero, while the power level of the excitation source is higher than the critical level; the duration of the output pulse being only a small fraction of the duration of the excitation pulse. This is referred to as early extinction or early termination.

The complex interactions which occur in the dye molecule after excitation and which give rise to early extinction of the laser output are not fully understood. At present it is known that dye molecules which have absorbed energy and been excited from the ground state to the excited singlet state may return directly to the ground state giving rise to stimulated emission which is governed by the lifetime of the dye molecule in the excited singlet state..

The dye molecule may return to the ground state by a second process involving intersystem crossing. In this process, the dye molecules relax to an intermediate energy level, or lowest triplet state, which lies between the excited singlet state and the ground state. The excited molecule will then return to the ground state; decaying in a process which may give rise/to radiation known as phosphorescence and which does not contribute to the laser output.

The lifetime of the dye molecules in the lowest triplet state is generally much longer than the lifetime of the dye molecules in the excited singlet state. Under excitation, molecules tend to accumulate in this triplet state resulting in a depletion of the molecules in the ground state. Consequently fewer molecules are available for excitation.

The molecules in the lowest triplet state also may absorb energy and make a transition to a higher triplet state. The wavelength region of the strongest absorption of the dye molecules in the lowest triplet state may be near the wavelength of the laser emission. Thus dye molecules in the lowest triplet state tend to absorb the laser emission causing a large optical loss at the wavelengths for which laser emission is most probable.

To circumvent the problem of early extinction, a dye laser can be excited with an extremely short duration excitation pulse such as provided by a ruby laser. With a pulse of this duration, the laser threshold is reached and emission occurs before a significant number of molecules can accumulate in the triplet states. However, the laser output pulse is also of short duration.

To produce a dye laser capable of providing longer output-pulse duration or continuous operation requires the efficient quenching of the triplet state lifetime of the dye molecule. Triplet state quenchers for gases have been known for some time. One of these, molecularoxygen, has also been shown to act as a triplet state quencher for a dye. Molecular oxygen has been added to solution containing the dye Rhodamine 6G to produce an active medium for a dye laser. When this active medium is excited, an output pulse is obtained which has a longer duration than the laser output pulse obtained from a Rhodamine 6G solution without the addition of oxygen. However, while the duration of the output pulse has been increased by the addition of oxygen, the output pulse is still only a small fraction of the duration of the excitation pulse. Oxygen also can have a detrimental effect on certain dyes by efiiciently quenching the excited singlet state as well as the triplet state lifetime and by causing dye photolysis. In addition, dissolved oxygen or other dissolved gases in solution may degrade the optical quality of the laser medium and cause dynamic losses which reduce the laser output power. Accordingly, we have invented an active medium for dye lasers wherein the problem of early extinction is reduced while distortion of the laser output beam is maintained relatively low.

SUMMARY OF THE INVENTION This invention relates to dye lasers and in particular to dye lasers having an active material comprising an organic dye and a triplet state quencher.

The active material comprises a dye, Rhodamine 6G, and liquid cycloheptatriene, CqHg (hereinafter designated CHT) dissolved in a solvent. The sol-vent can be an alcohol, typically ethanol or methanol. This liquid active material when excited by electromagnetic radiation emits a laser output pulse which has a longer duration and higher energy than the output pulse of a similar liquid active material without CHT. Since CHT is a liquid, the distortion of the laser output is lower than the distortion which occurs when the active medium utilizes a dissolved gas such as 0 as a triplet state quencher. Further features and advantages of the present invention will become more readily apparent from the following description of specific embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT The present active medium for a dye laser contains a dye Rhodamine 6G and the triplet state quencher, CHT, dissolved in a solvent. The solvent is typically an alcohol such as ethanol or methanol.

CHT is an unsaturated cyclic hydrocarbon having the general formula C H with molecular weight equal to 92 and structure as given in FIG. 1. It may be synthesized using a method described in an article entitled A New Synthesis of Cycloheptatriene by H. L. Drydan, Jr. which appear at 76 Journal of the American Chemical Society, 2841 (1954). Briefly, bicyclo[3.2.0] hept-Z-ene-G-one is reduced with lithium aluminum hydride which yields bicyclo[3.2.0] hept-2-ene-6-ol (I). Treatment of (I) with methanesulfonyl chloride in pyridine yields methanesulfonate (II) as a crude oil which is thermally unstable. Solvolysis of (II) in acetic acid containing preferably 2 moles equivalents of sodium dihydrogen phosphate monohydrate yields cycloheptatriene.

In the preparation of a dye medium of this invention, a solution having a desired dye concentration is produced by stirring a measured amount of Rhodamine 6G in a solvent until the dyes are fully dissolved. CHT is then added to the dye solution. For example, to obtain a solution having a concentration of approximately 5 X 10- moles per liter of solution (m./l.) of Rhodamine 6G and approximately 4.5 lm./l. of CHT, 25 milligrams of Rhodamine 6G are dissolved in 1 liter of ethanol. When the dye is fully dissolved, 4 cubic centimeters of CHT are added to produce the final solution. By similar methods, laser media having various concentrations of Rhodamine 6G and CHT can be made.

A dye laser utilizing the active medium prepared in the above described manner is shown in FIG. 2. Laser 10 is shown comprising a hollow cylindrical cell 12 which may be made of any material which transmits light at the excitation wavelength of the active material, typically quartz or Pyrex. The ends of cell 12 are sealed by optically flat, parallel end faces 14 and 16 having reflective coatings thereon; one, for example 14, being totally reflective while the other, 16 being only partially reflective. A typical cell is three inches long and has an inside diameter of one-eighth A2) inch.

The laser is tilled with the active material. A pump 18 is provided to circulate the active material within the laser and a heat exchanger 20 may be provided to cool the material as it leaves the cell. .A flash lamp 22, coupled to an external trigger circuit (not shown) provides excitation energy for the laser.

In operation, the dye laser solution flows through cell 12 wherein it is excited by absorbing energy emitted by flash lamp 22. The absorbed energy raises the dye molecules from the ground state to the excited singlet state. When critical inversion is reached laser emission occurs and the molecules lose energy and return to the ground state. Quenching of the triplet-state of the dye molecule is believed to occur by the following energy-exchange process: Dye molecules in the lowest triplet state collide with molecules of CHT whereby energy is transferred from the dye molecules to the CHT molecules. The CHT molecules are thereby raised to an excited triplet state while the dye molecules return to the ground state where they are available for reexcitation. The CHT molecules thereafter lose energy and return to a lower energy level. However this process does not interfere with the laser emission.

The laser may also have a filter 24 positioned between flash lamp 22 and cell 12. Filter 24 is designed to block the ultraviolet (UV) component (wavelengths below 4000 angstroms) of the excitation energy and transmit the remaining energy. Experiments have shown that the laser pulse duration and output power are increased when the excitation energy is filtered in this manner. It is believed that energy in the ultraviolet region absorbed by the CHT molecules raises their energy from the ground state to the excited singlet state where they are no longer available for quenching the triplet state of the dye molecules. The elimination of the 'UV radiation effects a relative increase in the concenrtation of CHT molecules in the ground state resulting in more eflicient quenching.

Experiments were carried out utilizing the dye laser medium in a system comprising a laser similar to that shown in FIG. 2. For the purposes of these experiments a photodiode 26 is positioned in the path of the laser output beam. The photodiode output is applied to an oscilloscope for measurement of the energy and duration of the laser output beam. The table below shows the improvement in lasing parameters due to the addition of CHT to dye solutions.

IMPROVEMENT IN LASING PARAMETERS DUE TO CHT [Rhodamine 6G concentration: 5 10- m./l.]

Arbitrary units. Lasing does not occur.

The output of the dye laser was compared, for various concentrations of CHT, with the air-equilibrium and nitrogen-saturated solutions for a constant concentration of dye. The experiments were carried out as follows: A dye solution of Rhodamine 6G was prepared. The oxygen content of this solution is the amount of air in equilibrium with the solvent at ambient temperature and pressure and is referred to as the air equilibrium solution. The laser was excited and the results indicated in the air-equilibrium line in the table. The oxygen was then removed from all the solutions by saturation with nitrogen gas. The laser was excited. However no output was obtained as indicated in the nitrogen saturated line in the table. CHT was then added in various concentrations and the laser was again excited. The results at certain values of CHT concentrations are given in the table. For each test run, the oscilloscope settings were maintained constant. When a laser output occurred an oscilloscope trace was produced and recorded. The area under these curves provides a quantitative comparison of the output energy of the laser under the different test conditions. These are the values recorded in the output energy lines of the table.

The improved laser output brought about by the addition of CHT to the dye solution is evident from the table. Thus, the addition of CHT to a dye solution of Rhodamine 66 increased the energy of the laser output almost 200 times over the air equilibrium dye laser solution while the duration of the output pulse approximately doubled.

It is preferred that the dye concentration be in the range of 10- to 10* m./l. Below a dye concentration of 10- m./l. the gain in the dye medium is insufficient to overcome the losses of the laser and no useful laser output is obtained. At dye concentrations above 10 m./l. sufficient energy can not be pumped into the laser to cause a laser output.

The concentration of CHT in solution is chosen so that the laser output is enhanced. Rhodamine 6G requires a minimum concentration of about 2X 10- As the CHT concentration is increased there generally appears to be an improvement in the laser output up to a concentration of about 7X l m./l. Above this concentration there is a degradation of the laser output beam. However a laser output can be obtained with the concentration of CHT at the limit of solubility of the CHT in the solution.

What is claimed is:

1. An active material for a liquid laser which comprises a solution containing Rhodamine 6G and cycloheptatriene dissolved in a solvent.

2. The active material of claim 1 wherein the concentration of said Rhodamine 6G is in the range of 10 m./l. of solution to 10- m./l. of solution.

3. The active material of claim 2 wherein the concentration of cycloheptatriene is at least suificient to enhance the laser output over that obtained without cycloheptatriene.

4. The active material of claim 1 wherein the concentration of cycloheptatriene is in the range 2 10 m./l. of solution to 8 10- m./l. of solution.

5. The active material of claim 1 wherein the concentration of Rhodamine 6G is 10- m./l. of solution and Pappalardo et al.: Applied Physics Letters, 16, 267 (1970).

ROBERT F. BURNETT, Primary Examiner L. T. KENDELL, Assistant Examiner US. Cl. X.R. 

